Far-Infrared Emission from Intracluster Dustin Abell Clusters

Abstract

Strip scanning measurements with crossing position angleshave been carried out at 120 μm and 180 μmwith the ISOPHOT instrument aboard ISO to observe extended FIRemission from six Abell clusters. Theraw I_{120 μm}/I_{180 μm} surface brightness ratioincluding zodiacal light shows a bump towards Abell 1656 (Coma), dipstowards Abell 262 and Abell 2670, and is without an unambiguousfeature towards Abell 400, Abell 496, and Abell 4038. The subtractionof the zodiacal light leaves the bump towards Abell 1656 stillpresent, while the dips towards Abell 262 and Abell 2670 are no longerdiscernible. This behavior can be reconciled in Abell 1656 with alocalized excess of emitting material outside the Galaxy withproperties different from the galactic foreground cirrus, and in Abell262 and Abell 2670 with galactic cirrus structures localized on theline-of-sight to these clusters. At 120 μmthe excesstowards Abell 1656 (Coma) is ≈ 0.2 MJy/sr, with an integratedexcess flux within the central region of 10'-15' diameter of ≈ 2.8 Jy. It is interpreted asbeing due to thermal emission from intracluster dust distributed inthe hot X-ray emitting intracluster medium. Only a rough estimate ofthe associated dust mass of M_D ≈ 10⁷ M_⊙ can be derived, since the dust temperature is poorly constrained. Thevisual extinction associated with this dust mass is negligible(A_V << 0.1 mag) and much smaller than claimed fromoptical observations. No evidence for intracluster dust is found inthe other five clusters observed. The rather low inferred dust massin Abell 1656 together with the absence of a signature forintracluster dust in the other clusters observed indicatesthatintracluster dust is likely not responsible for the excess X-rayabsorption seen in cooling flow clusters. These observations therebyrepresent a further unsuccessful attempt in detecting the presumed final stage of the cooling flow material, in accord with quite a number of previous studies. Finally, the observed dimming of the high - redshift supernovae is unlikely be attributable to extinction causedby dust in the intracluster or even a presumed intercluster medium.